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Instrument could be used to validate other Earth observing missions.

This passive laser heterodyne radiometer (LHR) instrument simultaneously
measures multiple trace gases in the
atmospheric column including carbon
dioxide (CO2) and methane (CH4), and
resolves their concentrations at different
altitudes. This instrument has been
designed to operate in tandem with the
passive aerosol sensor currently used in
AERONET (an established network of
more than 450 ground aerosol monitoring
instruments worldwide). Because
aerosols induce a radiative effect that
influences terrestrial carbon exchange,
simultaneous detection of aerosols with
these key carbon cycle gases offers a
uniquely comprehensive measurement
approach.

Laser heterodyne radiometry is a technique
for detecting weak signals that was
adapted from radio receiver technology.
In a radio receiver, a weak input signal
from a radio antenna is mixed with a
stronger local oscillator signal. The
mixed signal (beat note, or intermediate
frequency) has a frequency equal to the
difference between the input signal and
the local oscillator. The intermediate frequency
is amplified and sent to a detector
that extracts the audio from the signal.

In the LHR instrument described here,
sunlight that has undergone absorption
by the trace gas is mixed with laser light at
a frequency matched to a trace gas
absorption feature in the infrared (IR).
Mixing results in a beat signal in the RF
(radio frequency) region that can be
related to the atmospheric concentration.
For a one-second integration, the estimated
column sensitivities are 0.1 ppmv for
CO2, and <1 ppbv for CH4.

The only network that currently measures
CO2 and CH4 in the atmospheric column
is TCCON (Total Carbon Column
Observing Network), and only two of its
16 operational sites are in the United
States. TCCON data is used for validation
of GOSAT data, and will be used for
OCO-2 validation. While these Fourier-transform
spectrometers (FTS) can measure
the largest range of trace gases, the
network is severely limited due to the high
cost and extreme size of these instruments
(these occupy small buildings and require
personnel for operation). The
LHR/AERONET in strument offers a significantly
smaller (carry-on luggage size)
autonomous instrument that can be
incorporated into AERONET’s much
larger (450 instruments) global network.

This work was done by Emily Steel and
Matthew McLinden of Goddard Space Flight
Center. GSC-16327-1

This Brief includes a Technical Support Package (TSP).

Miniaturized Laser Heterodyne Radiometer (LHR) for Measurements of Greenhouse Gases in the Atmospheric Column (reference GSC-16327-1) is currently available for download from the TSP library.

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